It has long been known in the packaging industry to employ packaging materials comprising outer layers of thermoplastic, these outer layers being capable of being sealed to one another by fusion after heating and compression for the formation of tight and durable sealing joints or seams. One form of package production employs so-called blanks which are filled and sealed at the top, or alternatively the bottom, and then finally formed. These prefabricated blanks are cut to exact dimensions in order to obtain packages as cheap as possible, with the result that that region which is to be sealed after the filling operation consists of a very narrow area. When the two edges which are to be sealed to one another are compressed together, it may happen that they are not entirely in register with one another, which even further reduces that area on which the sealing is to be effected. Possibly, the sealing edge may also include parts which may not under any circumstances be overstepped, such as, for example crease lines or the like.
Normally, the sealing of filled blanks is carried out using hot air. This requires a large number of mechanical parts which, when the intention is to produce aseptic packages, constitutes a disadvantage simply because there are more parts which must be kept aseptic.
Aseptic packages are commonly made from a packaging material which contains a thin metal foil, preferably aluminium foil. When the packaging material contains a metal foil, it is possible to employ induction welding, which is also a known technique within the packaging industry.
The technique for induction welding is based on the fact that the magnetic field surrounding a conductor impressed with an alternating current is capable of inducing a current in an adjacent electrically conductive material, which, depending upon the resistance of the material, entails a more or less manifest heating of the material. Hence, in induction welding an induction coil is applied of the same configuration as the desired finished weld adjacent the laminate containing aluminium foil, whereafter the laminate is compressed with the material with which it is to be united. The aluminium foil will now be heated in a pattern corresponding to the configuration of the coil and, by suitable selection of the current force, frequency and processing time, the material can be heated to the desired temperature, i.e. a temperature which is sufficiently elevated for adjacent layers of the thermoplastic material to be heated to a fusion, or sealing temperature, whereupon thermoplastic layers brought together under pressure are caused to fuse together for obtaining a tight and durable seal or seam.
However, existing inductors are most often designed for welding on a continuous web in which the transverse seams are made two-by-two with an intermediate spacing where the web is then separated. In order to be able to use existing inductors on a pre-cut blank, with the restricted space available for the sealing joint or seam, it is possible to make inductors with narrower poles, but this generally entails problems in cooling the inductor and, moreover, such inductors are also more difficult to manufacture. The space between the two conductors in the inductor may also be made narrower, but then a greater current power will be required to obtain a reliable sealing seam, and this in its turn implies greater losses in the inductor.